The development of a hydrophilic water wettable membrane is necessitated by numerous filtration applications that use water, or water containing additives as the permeate. This includes filtration in the microfiltration (0.1-10 .mu.m) and the ultrafiltration (0.001-0.01 .mu.m) range.
Polysulfones (PS) and polyethersulfones (PES) are a class of hydrophobic polymers widely used today in the manufacture of flat sheet and hollow fiber membranes, which operate in the microfiltration range and the ultrafiltration range. Engineering plastics, such as polysulfones and polyethersulfones are widely used due to their easy processability and their ability to produce membranes having a wide range of pore structures. Moreover, they possess good thermal stability and good resistance to acid and alkali. However, both PS and PES are inherently hydrophobic polymers and their applications in microfiltration and to a larger extent in ultrafiltration is somewhat limited by the difficulties encountered in wetting such membranes.
The pressure required for one fluid to displace another fluid in the pores of a membrane (.DELTA.P) is related to the pore size and the interfacial tension of the contacting liquids by the relation, EQU .DELTA.P=-2.gamma.cos .theta./r.sub.p
where r.sub.p is the pore radius, .gamma. is the interfacial tension between the two fluids, and .theta. is the contact angle of the liquid on the membrane. The minimum pressure required to make the membrane permeable depends on the membrane material, the permeant liquids and the pore size of the membrane. If the fluid in the membrane pores is air, then the interfacial tension is that of the fluid/air interface. The inverse relationship between the pore radius and the applied pressure, coupled with the high contact angle of the liquid on the membrane makes the pressure required for water to wet hydrophobic membranes very high.
At the high pressures required to wet some ultrafiltration membranes, compression of the membranes may lead to an irreversible collapse of the pore structure and a loss of hydraulic permeability. Hydrophobic membranes like PS and PES are also prone to non-specific protein adsorption by virtue of their large hydrophobic surfaces. In pharmaceutical and therapeutic applications this may lead to rapid blockage of the pores' diameters and fouling of the membrane.
One obvious solution to the above problem with hydrophobic polymers is to use hydrophilic polymers as membrane forming materials. However, such hydrophilic polymers, like cellulose, are limited in their use due to their poor chemical resistance and lack of processability.
Several efforts have been made in the prior art to modify the hydrophobic properties of membranes made from engineering plastics. These have included chemical modifications of pre-formed membranes, and the use of hydrophilic polymers as additives in the membrane forming process. Examples of chemically modifying preformed membranes include, plasma treatment of the membranes to introduce hydrophilic groups on the surface, deposition of thin coatings of hydrophilic polymers on the surface of the hydrophobic membranes and the addition of hydrophilic polymers to the cast solution.
Additives used in membrane formation have covered a wide range of polymers. Water soluble polymers, such as polyethylene glycol (PEG) and polyvinyl pyrrolidone (PVP) have been used mainly in the prior art as pore formers in the manufacture of porous PS and PES membranes. Despite their success as pore formers, some portion of the foregoing additives remain in the membrane conferring a lower interfacial tension to the membrane. Examples of preparing hydrophilic membranes by using hydrophilic polymers in the casting solution as additives include the use of polyvinyl pyrrolidone as described in U.S. Pat. No. 4,051,300 to Klein et al., and the use of polyethylene glycols as described in Japanese Patent No. 54-26283, and in U.S. Pat. No. 4,900,449 to Kraus et al.
In both these methods, a small amount of the hydrophilic water soluble additive is retained in the membrane, and is susceptible to leach out on prolonged use in aqueous environments. In addition to the above problem of leaching, the Kraus et al. reference is useful only for polyethersulfone membranes and does not produce wettable membranes with polysulfone membranes. Other notable references using similar approaches (vis addition of PVP and/or PEG additives) include U.S. Pat. Nos. 5,232,597 to Eguchi and 5,340,480 to Kawata et al.
The use of coatings made from hydrophilic polymers, with or without subsequent crosslinking of the coating, is of limited applicability for ultrafiltration membranes, because the membranes pores shrink during the curing of the membranes (See; U.S. Pat. No. 5,277,812 to Hu et al.) Also the polymer, polyethylene imine (Corcat-600) is used as a membrane coating, producing a membrane which has ion exchange characteristics. Another example of a charge modified hydrophilic membrane, using an epichlorohydrin modified polyamine, is disclosed in U.S. Pat. No. 5,269,931 to Hu et al. While such crosslinked coatings may be stable to water extractions, they modify the pore sizes of the membranes. In the case of very small pores, such as needed for ultrafiltration applications, they lead to a wide range of pore sizes which can be difficult to control during manufacturing.
Other approaches of producing a membrane having a wettable surface include the use of additives that are not necessarily water soluble. An example of such an approach is disclosed in U.S. Pat. No. 5,178,765 to Hu et al. Specifically, this reference discloses a blended polyethersulfone membrane which contains a poly(2-alkyl or aryl) 2-oxazoline resin and an excess of PVP.
A permanently charged membrane containing a guarternized nitrogen containing polymer is described in U.S. Pat. No. 5,114,585 to Kraus et al.
U.S. Pat. No. 5,076,935 to Kraus et al. describes a membrane which contains a blend of polyethersulfone/phenoxy resin.
U.S. Pat. No. 4,961,852 to Pemawansa et al. describes a coating of a polyaldehyde polymer on PES. The polyaldehyde coating introduces hydrophilicity to the PES polymer, but it contains reactive aldehyde groups and is not useful for filtration involving reactive functional groups.
U.S. Pat. No. 5,158,721 to Allegrezza, Jr. et al. describes a membrane comprising a membrane forming hydrophobic polymer and a hydrophilic monomer; which is cast, cured and coagulated before drying. U.S. Pat. No. 5,137,633 to Wang also describes the use of crosslinked polyamine epichlorohydrin resin, and a monomer precursor crosslinked by radical polymerization, to obtain a charged resin.
Introduction of sulfonic acid groups in pre- or post-formation of the membranes has been employed in the prior art to improve water wettability of PS and PES membranes (See; for example, U.S. Pat. No. 3,885,122 to Bourgnel and German Patent No. 2,829,630). Despite their success in providing water wettability to PS and PES polymers, introduction of ionic sulfonic acid groups has the distinct disadvantage of the membrane exhibiting ion-exchange properties.
U.S. Pat. No. 5,071,448 to Bikson et al. deals with the use of sulfonated polysulfones to form semipermeable membranes.
The aforementioned techniques are generally only useful for microporous membranes. However, when applied to ultrafiltration membranes, these coatings do not penetrate the pores to make them wettable, but instead form coatings essentially blocking the smallest and most numerous pores. Other prior art approaches to modifying these membranes also suffer from the drawbacks discussed above.
In view of the numerous drawbacks mentioned hereinabove, it would be highly advantageous, to develop a permselective membrane which can be used in hemodialysis, ultrafiltration and microfiltration applications that has a high degree of wettability.